Toners for development of an electrostatic image are conventionally produced by melt kneading of a pigment, resin and other toner ingredients, followed by pulverisation. Classification is then needed to generate an acceptably narrow particle size distribution.
Recently attention has been focussed on chemical routes to toners, where a suitable particle size is not attained by a milling process, which avoid the need for a classification step. By avoiding the classification step, higher yields can be attained, especially as the target particle size is reduced. Lower particle size toners are of considerable interest for a number of reasons, including better print resolution, lower pile height, greater yield from a toner cartridge, faster or lower temperature fusing, and lower paper curl.
Several routes to chemical toners have been exemplified. These include suspension polymerisation, solution-dispersion processes and aggregation routes. Aggregation processes offer several advantages including the generation of narrow particle size distributions, and the ability to make toners of different shape. The toner shape is particularly important in toner transfer from the organic photoconductor (OPC) to the substrate, and in cleaning of the OPC by a blade cleaner.
Several aggregation processes have been reported. U.S. Pat. No. 4,996,127 (Nippon Carbide) reports a process in which black toner particles are grown by heating and stirring resin particles made by emulsion polymerisation with a dispersion of carbon black, where the resin contains acidic or basic polar groups. Numerous patents from Xerox (e.g. U.S. Pat. No. 5,418,108) describe a flocculation process where particles stabilised by anionic surfactants are mixed with particles stabilised by cationic surfactants (or where a cationic surfactant is added to particles stabilised by an anionic surfactant). U.S. Pat. No. 5,066,560 and U.S. Pat. No. 4,983,488 (Hitachi Chemical Co.) describe emulsion polymerisation in the presence of a pigment, followed by coagulation with an inorganic salt, such as magnesium sulphate or aluminium chloride. The applicants' own patent applications WO 98/50828 and WO 99/50714, describe aggregation processes in which a surfactant used to stabilise the latex (i.e. the aqueous dispersion of the resin) and pigment is converted by a pH change from an ionic to a non-ionic state, so initiating flocculation.
To form a permanent image on the substrate, it is necessary to fuse or fix the toner particles to the substrate. This is commonly achieved by passing the unfused image between two rollers, with at least one of the rollers heated. It is important that the toner does not adhere to the fuser rollers during the fixation process. Common failure modes include paper wrapping (where the paper follows the path of the roller) and offset (where the toner image is transferred to the fuser roller, and then back to a different part of the paper, or to another paper sheet). One solution to these problems is to apply a release fluid, e.g. a silicone oil, to the fuser rollers. However this has many disadvantages, in that the oil remains on the page after fusing, problems can be encountered in duplex (double-sided) printing, and the operator must periodically re-fill the oil dispenser. These problems have led to a demand for so-called “oil-less” fusion, in which a wax incorporated in the toner melts during contact of the toner with the heated fuser rollers. The molten wax acts as a release agent, and removes the need for application of the silicone oil.
There are many problems associated with the inclusion of wax in a toner. Wax present at the surface of the toner may affect the triboelectric charging and flow properties, and may reduce the storage stability of the toner by leading to toner blocking. Another problem frequently encountered is filming of the wax onto the metering blade and development rollers (for mono-component printers) or the carrier bead (for dual-component printers or copiers), and onto the photoconductor drum. Where contact charging and/or contact development are employed, and where cleaning blades or rollers are used, these can place an extra stress on the toner and make it more prone to filming. If the wax is not well dispersed in the toner problems with transparency in colour toners can be found, and high haze values result. With conventional toners, prepared by the extrusion/pulverisation route, it has only proved possible to introduce relatively small amounts of wax without encountering the above problems.
With colour toners, the demands on the toner to achieve oil-less release are much more severe than with monochrome printing. As typically four colours are used in full-colour printing, the mass of toner which can be deposited per unit area is much higher than with black printing. Print densities of up to around 2 mg/cm2 may be encountered in colour printing, compared with about 0.4-0.7 mg/cm2 in monochrome prints. As the layer thickness increases it becomes more difficult to melt the wax and obtain satisfactory release at acceptable fusion temperatures and speeds. Of course it is highly desirable to minimise the fusion temperature, as this results in lower energy consumption and a longer fuser lifetime. With colour printing it is also important that prints show high transparency. In addition it is necessary to be able to control the gloss level. Inclusion of waxes in colour toners can have detrimental effects on transparency, and can make it difficult to reach higher gloss levels.
The efficiency of wax melting can be increased by reducing the wax melting point. However this often leads to increased storage stability problems, and in more pronounced filming of the OPC or metering blade. The domain size of the wax is also important, as this affects the release, storage stability and transparency of the toner.
The release properties of the toner can also be affected by the molecular weight distribution of the toner, i.e. the resin thereof. Broader molecular weight distribution toners, which include a proportion of higher molecular weight (or alternatively cross-linked resin), generally show greater resistance to offset at higher fusion temperatures. However, when large amounts of high molecular weight resins are included, the melt viscosity of the toner increases, which requires a higher fusion temperature to achieve fixation to the substrate and transparency. The haze values of the prints will then vary considerably with fusion temperature, with unacceptably high values at low fusion temperatures. Haze may be assessed using a spectrophotometer, for example a Minolta CM-3600d, following ASTM D 1003.
Therefore the requirements for achieving an oil-less fusion colour system are severe. It is necessary to achieve a reasonably low fusion temperature, with an acceptably wide release temperature window, including with high print densities. The prints must show good transparency with controllable gloss. The toner must not show blocking under normal storage conditions, and must not lead to filming of the OPC or metering blade.
In addition it is important that the quality of the prints is maintained over a long print run, and that the toner is efficiently used. To achieve these goals there must be little development of the non-image areas of the photoconductor (OPC) and the toner must show a high transfer efficiency from the photoconductor to the substrate (or to an intermediate transfer belt or roller). If the transfer efficiency is close to 100% it is possible to avoid the need for a cleaning step, where residual toner is removed from the photoconductor after transfer of the image. However many electrophotographic devices contain a mechanical cleaning device (such as a blade or a roller) to remove any residual toner from the photoconductor. Such residual toner may arise either from development of the non-image areas of the photoconductor, or from incomplete transfer from the photoconductor to the substrate or intermediate transfer belt or roller. A high transfer efficiency is especially important for colour devices, where sometimes more than one transfer step is required (for example from the photoconductor to a transfer belt or roller, and subsequently from the transfer belt or roller to the substrate).
It is known in the art that the shape of the toner can have a pronounced effect on its transfer and cleaning properties. Toners prepared by conventional milling techniques tend to have only moderate transfer efficiencies due to their irregular shape. Spherical toners may be prepared by chemical routes, such as by suspension polymerisation or by latex aggregation methods. These toners can transfer well, but the efficiency of cleaning with mechanical cleaning devices such as cleaning blades is low.
It is therefore desirable to produce a toner which can satisfy many requirements simultaneously. The toner should be capable of fixing to the substrate at low temperatures by means of heated fusion rollers where no release oil is applied. The toner should be capable of releasing from the fusion rollers over a wide range of fusion temperatures and speeds, and over a wide range of toner print densities. To achieve this it is necessary to include a wax or other internal release agent in the toner. This release agent must not cause detrimental effects on storage stability, print transparency or toner charging characteristics, and must not lead to background development of the photoconductor (OPC). It must also not lead to filming of the metering blade or development roller (for a mono-component device) or the carrier bead (for a dual-component device), or of the photoconductor. In addition the shape of the toner must be controlled so as to give high transfer efficiency from the photoconductor to the substrate or intermediate transfer belt or roller, and from the transfer belt or roller (where used) to the substrate. If a mechanical cleaning device is used the shape of the toner must also be such as to ensure efficient cleaning of any residual toner remaining after image transfer.
Several patents exemplify aggregation processes where a single latex, made by a one-stage emulsion polymerisation process, is aggregated with a wax dispersion. Examples where a system based on counterionic surfactants (i.e. an anionic and a cationic surfactant) is used include U.S. Pat. No. 5,994,020 and U.S. Pat. No. 5,482,812 (both to Xerox). Examples where an inorganic coagulant is used include U.S. Pat. No. 5,994,020, U.S. Pat. No. 6,120,967, U.S. Pat. No. 6,268,103 and U.S. Pat. No. 6,268,102 (all to Xerox). Mixed inorganic and organic coagulants are used in U.S. Pat. No. 6,190,820 and U.S. Pat. No. 6,210,853 (both to Xerox). U.S. Pat. No. 4,996,127 (Nippon Carbide) exemplifies a process in which a latex containing an acidic-functional group is heated and stirred with a wax dispersion and carbon black to grow aggregate toner particles.
U.S. Pat. No. 5,928,830 (Xerox) discloses a two stage emulsion polymerisation to make a core shell latex. The shell is made generally of higher molecular weight and/or Tg than the core. The latex is then mixed with pigment and flocculated through use of counterionic surfactants. Inclusion of wax is not exemplified.
U.S. Pat. No. 5,496,676 (Xerox) discloses use of blends of different latexes with different molecular weight to increase the fusion latitude. Each latex is made by a single stage polymerisation. Toners were made by flocculating the mixed latexes with a pigment dispersion containing a counterionic surfactant. Inclusion of wax is not exemplified.
In U.S. Pat. No. 5,965,316 (Xerox) encapsulated waxes are made by carrying out the emulsion polymerisation in the presence of a wax dispersion. These emulsion polymers containing wax are mixed with non wax containing latexes of similar molecular weight, and toners made using a counterionic flocculation route.
JP 2000-35690 and JP 2000-98654 describe aggregation processes where a non-ionically stabilised dispersion of an ester-type wax is aggregated with mixed polymer emulsions of different molecular weight.
U.S. Pat. No. 5,910,389, U.S. Pat. No. 6,096,465 and U.S. Pat. No. 6,214,510 (Fuji Xerox) disclose blends of resins with different molecular weights, incorporating hydrocarbon waxes of melting point ˜85° C. U.S. Pat. No. 6,251,556 (Fuji Xerox) also discloses blends of resins, as well as a two stage emulsion polymerisation to make a core shell latex. The only wax which is incorporated is a high melting point (160° C.) polypropylene wax.
Control over the toner particle shape in aggregation processes has been demonstrated. U.S. Pat. No. 5,501,935 and U.S. Pat. No. 6,268,102 (Xerox) both exemplify spherical particles. Toners which are non-spherical, but have low shape factors are disclosed in U.S. Pat. No. 6,268,103 (Xerox); U.S. Pat. No. 6,340,549, U.S. Pat. No. 6,333,131, U.S. Pat. No. 6,096,465, U.S. Pat. No. 6,214,510 and U.S. Pat. No. 6,042,979 (Fuji Xerox); and U.S. Pat. No. 5,830,617 and U.S. Pat. No. 6,296,980 (Konica). Advantages of lower shape factors in improving transfer efficiency are shown in U.S. Pat. No. 6,214,510 and U.S. Pat. No. 6,042,979 (Fuji Xerox) and U.S. Pat. No. 5,830,617 (Konica). Other references which disclose shape factors of toners are U.S. Pat. No. 5,948,582, U.S. Pat. No. 5,698,354, U.S. Pat. No. 5,729,805, U.S. Pat. No. 5,895,151, U.S. Pat. No. 6,308,038, U.S. 5,915,150 and U.S. Pat. No. 5,753,396. However, none of these references discloses a toner for use in a mono-component electroreprographic apparatus which is capable of demonstrating: release from oil-less fusion rollers over a wide range of fusion temperature and print density; high transparency for OHP slides over a wide range of fusion temperature and print density; high transfer efficiency and the ability to clean any residual toner from the photoconductor, and the absence of filming of the metering blade, development roller and photoconductor over a long print run.